1. Field of the Invention
This invention generally relates to optical communication systems and more particularly to an optical transmitter.
2. Description of the Related Art
The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
The T-1 standards committee of ANSI has provided a draft document, xe2x80x9cANSI T1.105-1988xe2x80x9d, dated Mar. 10, 1988, which sets forth specifications for rate and format of signals that are to be used in optical interfaces. The provided specifications detail the Synchronous Optical Network (SONET) standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals into a common medium. In essence, SONET established a uniform standardization transmission and signaling scheme, which provided a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
Currently this expansion of bandwidth is being accomplished by what is known as xe2x80x9cwavelength division multiplexingxe2x80x9d (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber datastreams on different portions of the light spectrum. WDM is therefore the optical equivalent of frequency division multiplexing (FDM). Current implementations of WDM involve as many as 128 semiconductor lasers each lasing at a specific center frequency within the range of 1525-1575 nm. Each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is combined onto a single optic fiber for transmission. As this digital signal is passed across a SONET network, it will be subject at various intervals to amplification by, for example, Erbium doped amplifiers and dispersion compensation by, for example, optical circulators with coupled Bragg filters. At each node in the network, e.g. central office or remote terminal, optical transceivers mounted on fiber line cards are provided. On the transmit side, a framer permits SONET framing, pointer generation and scrambling for transmission of data from a bank of lasers and associated drivers, with each laser radiating at a different wavelength. On the receive side, the incoming signals are separated into channels detected by photodetectors, framed and decoded.
As more and more optical signal equipment (transmitting, receiving, amplification, coherence and switching) is being designed and utilized, a need has arisen for short and intermediate range optical links to allow high speed data transfers within various components of a central office. Currently such links require the same expensive circuits and lasers used in long-range optical links. What is needed is a short-and intermediate-range high data rate optical communication system that does not require the cost and complexity of prior art devices and which preferably can be achieved with a reduced form factor compared with prior art devices.
Described embodiments provide a method and apparatus for transmission of optical communications. One embodiment provides an optical transmitter that includes a control circuit to enhance the stability of output power levels, a modulator circuit with precise impedance matching for high frequency performance, and an optical coupling mechanism that relaxes the alignment tolerances between the laser and the fiber, and reduces sensitivity of the gain medium to external optical feedback. These features allow the transmitter to deliver an optical output beam that can be modulated over a wide range of frequencies, duty cycles and amplitudes with very precise definition of the rising and falling edges of the waveform. In combination these features result in an optical transmitter that may be fabricated with relatively low cost and a reduced form factor when compared with prior art optical transmitters.
In an embodiment of the invention a semiconductor laser transmitter is disclosed. The semiconductor laser transmitter includes: a gain medium, an optical fiber, and a reflector. The gain medium generates an emission along a path. The optical fiber includes a tip portion located in the path to optically couple with the emission. A reflector is located on the tip portion to limit an optical coupling efficiency of said gain medium with said optical fiber.
In an alternate embodiment of the invention a method for transmitting optical signals is disclosed. The method includes the acts of:
generating an optical emission along a path;
coupling an optical fiber with said optical emission along the path;
reflecting a portion of the optical emission away from the path;
generating a difference signal corresponding to a difference between a power level of the reflected portion of the optical emission and a reference power level; and
varying a power level of the optical emission generated in said first act of generating to correspond with the difference signal generated in said second act of generating to control the power level of the optical emission.